The invention relates to the field of televisions capable of displaying zoomed and/or cropped pictures, and in particular, to methods and apparatus for horizontally panning such zoomed or cropped pictures in televisions having a wide display format ratio screen. Most televisions today have a format display ratio, horizontal width to vertical height, of 4:3. A wide format display ratio corresponds more closely to the display format ratio of movies, for example 16:9. The invention is applicable to both direct view televisions and projection televisions.
Televisions having a format display ratio of 4:3, often referred to as 4.times.3, are limited in the ways that single and multiple video signal sources can be displayed. Television signal transmissions of commercial broadcasters, except for experimental material, are broadcast with a 4.times.3 format display ratio. Many viewers find the 4.times.3 display format less pleasing than the wider format display ratio associated with the movies. Televisions with a wide format display ratio provide not only a more pleasing display, but are capable of displaying wide display format signal sources in a corresponding wide display format. Movies "look" like movies, not cropped or distorted versions thereof. The video source need not be cropped, either when converted from film to video, for example with a telecine device, or by processors in the television.
Televisions with a wide display format ratio are also suited to a wide variety of displays for both conventional and wide display format signals, as well as combinations thereof in multiple picture displays. However, the use of a wide display ratio screen entails numerous problems. Changing the display format ratios of multiple signal sources, developing consistent timing signals from asynchronous but simultaneously displayed sources, switching between multiple sources to generate multiple picture displays, and providing high resolution pictures from compressed data signals are general categories of such problems. Such problems are solved in a wide screen television according to this invention. A wide screen television according to various inventive arrangements is capable of providing high resolution, single and multiple picture displays, from single and multiple sources having similar or different format ratios, and with selectable display format ratios.
Televisions with a wide display format ratio can be implemented in television systems displaying video signals both at basic or standard horizontal scanning rates and multiples thereof, as well as by both interlaced and noninterlaced scanning. Standard NTSC video signals, for example, are displayed by interlacing the successive fields of each video frame, each field being generated by a raster scanning operation at a basic or standard horizontal scanning rate of approximately 15,734 Hz. The basic scanning rate for video signals is variously referred to as f.sub.H, 1 f.sub.H, and 1.sub.H. The actual frequency of a 1 f.sub.H signal will vary according to different video standards. In accordance with efforts to improve the picture quality of television apparatus, systems have been developed for displaying video signals progressively, in a noninterlaced fashion. Progressive scanning requires that each displayed frame must be scanned in the same time period allotted for scanning one of the two fields of the interlaced format. Flicker free AA-BB displays require that each field be scanned twice, consecutively. In each case, the horizontal scanning frequency must be twice that of the standard horizontal frequency. The scanning rate for such progressively scanned or flicker free displays is variously referred to as 2 f.sub.H and 2H. A 2 f.sub.H scanning frequency according to standards in the United States, for example, is approximately 31,468 Hz.
Considerable signal processing of the main video signal is necessary to implement many of the display formats which are especially appropriate for a wide screen television. The video data must be selectively compressed and expanded, depending upon the desired format. In one case, for example, it is necessary to compress the 4.times.3 NTSC video by a factor of 4/3, or 4:3, to avoid aspect ratio distortion of the displayed picture. In the other case, for example, the video can be expanded to perform horizontal zooming operations usually accompanied by vertical zooming. Horizontal zoom operations up to 33% can be accomplished by performing compressions less than 4/3, for example 5/4. A sample interpolator is used to recalculate the incoming video to a new pixel positions because the luminance video bandwidth, up to 5.5 MHz for S-VHS format, occupies a large percentage of the Nyquist or fold over frequency, which is 8 MHz for a 1024 f.sub.H system clock.
The luminance data for the main signal is routed along a main signal path including a FIFO line memory for compressing (pausing) and expanding (repeating) the data and an interpolator for recalculating sample values to smooth the data. However, the relative positions of the FIFO and the interpolator are different for compression than for expansion. In accordance with an inventive arrangement, switches or route selectors reverse the topology of the main signal path with respect to the relative positions of the FIFO and the interpolator, avoiding the need for two main signal paths requiring two FIFOs and two interpolators. In particular, these switches select whether the interpolator precedes the FIFO, as required for compression, or whether the FIFO precedes the interpolator, as required for expansion. The switches can be responsive to a route control circuit which is itself responsive to a microprocessor.
An interpolator control circuit generates pixel position values, interpolator compensation filter weighting and clock gating information for the luminance data. It is the clock gating information which pauses (decimates) or repeats the FIFO data to allow samples not to be written on some clocks for effecting compression or some samples to be read multiple times for expansion. In order to process a 4/3 compression, for example, wherein 4/3 represents the ratio of the number of input samples to the number of output samples, every fourth sample can be inhibited from being written into the FIFO. The average slope of a ramp read out of the luminance FIFO is 33% steeper than the corresponding input ramp. Note that 33% less active reading time is required to read out the ramp as was required to write in the data. This constitutes the 4/3 compression. It is the function of the interpolator to recalculate the luminance samples being written into the FIFO so that the data read out of the FIFO is smooth, rather than jagged.
Expansions may be performed in exactly the opposite manner as compressions. In the case of compressions the write enable signal has clock gating information attached to it in the form of inhibit pulses for writing to the output FIFO. For expanding data, the clock gating information is applied to the read enable signal. This will pause the data as it is being read from the FIFO. The average slope of a ramp read out of the luminance FIFO is 33% more shallow than the corresponding input ramp for a 3/4 expansion or zoom. In this case it is the function of the interpolator, which follows the FIFO, to recalculate the sampled data from jagged to smooth after the expansion. In the expansion case the data must pause while being read from the FIFO and while being clocked into the interpolator. This is different from the compression case where the data is continuously clocked through the interpolator. For both cases, compression and expansion, the clock gating operations can easily be performed in a synchronous manner, that is, events can occur based on the rising edges of the 1024 f.sub.H system clock.
There are a number of advantages in this topology for luminance interpolation. The clock gating operations, namely data decimation and data repetition, may be performed in a synchronous manner. If a switchable video data path topology were not used to interchange the positions of the interpolator and FIFO, the read or write clocks would need to be double clocked to pause or repeat the data. The term double clocked means that two data points must be written into the FIFO in a single clock cycle or read from the FIFO during a single clock cycle. The resulting circuitry cannot be made to operate synchronously with the system clock, since the writing or reading clock frequency must be twice as high as the system clock frequency. Moreover, the switchable topology requires only one interpolator and one FIFO to perform both compressions and expansions. If the video path switching arrangement described herein were not used, the double clocking situation can be avoided only by using two FIFO's to accomplish both compression and expansion. One FIFO for expansions would need to be placed in front of the interpolator and one FIFO for compressions would need to be placed after the interpolator.
A circuit for compressing and expanding video data comprises a FIFO line memory and an interpolator. A timing circuit generates control signals for writing data into the line memory and for reading data from the line memory to compress and expand the data. The interpolator smooths the data compressed or expanded in the FIFO line memory. A switching network selectively establishes a first signal path in which the line memory precedes the interpolator for implementing the data expansion and a second signal path in which the interpolator precedes the line memory for implementing the data compression. The switching network is controlled according to selected display formats requiring compression or expansion, for example by a microprocessor.